The present invention relates to LCD (liquid crystal display) to be used as a displaying apparatus for images and textual information for OA (office automation) appliance and the like, and more particularly to a configuration of LCD of a high quality of active matrix system for constraining the (displaying quality) reduction caused due to leakage of light in the pixels of an array substrate and for increasing aperture ratio.
The displaying quality means uniformity of luminance distribution in display area. If amount of leakage of light varies sharply between two places in the display area, displaying quality is degraded. Usually, the display area is divided to form a plurality of sub areas.
Accordingly, if leakage of light varies in each sub area, luminance difference between the sub areas causes a visible line so that displaying quality degrades.
FIG. 8 is an observing view of LCD of the active matrix system when the conventional size of display area is small. FIG. 9 is a sectional view of the pixel portion.
In FIG. 8, reference numeral 100 is an LCD, reference numeral 101 is a view-point of observer, and reference numeral 102 is a viewing angle .theta..
In FIG. 9, reference numeral 10 is a first glass substrate which is a base member for an array substrate, reference numeral 11 is a source bus line formed on the surface of first glass substrate 10, reference numeral 14 is a pixel electrode which is a transparent conductive film such as ITO (indium tin oxide) or the like formed on the surface of first glass substrate 10, and reference numeral 16 is a polyimide thin film serving as an alignment layer.
An array substrate 70 is composed of first glass substrate 10, source bus line 11, gate bus line 12 (not shown) and pixel electrode 14.
Reference numeral 20 is a second glass substrate serving as a base member of a color filter substrate arranged opposite to array substrate 70, reference numeral 21 is a black matrix which is a light shield portion formed by patterning, a metal film of Cr or the like on the surface of second glass substrate 20 in lattice shape using photo-lithography, reference numeral 22 is a common electrode, reference numeral 23 is a colored layer of red, blue or green formed on the surface of second glass substrate 20, reference numeral 24 is an alignment layer which is a polyimide thin film, reference numeral 30 is a liquid crystal layer, and reference numeral 25 is a liquid crystal molecule of liquid crystal layer 30.
A color filter substrate 80 is composed of second glass substrate 20, black matrix 21, and colored layer 23.
Reference numeral 90 is a polarizer arranged on the surface of each of array substrate 70 and color filter substrate 80.
Reference numeral 40 is a clearance between pixel electrode 14 and source bus line 11, reference numeral 60 is leakage of light, showing an area where the leakage of light of the illuminating light is recognized from clearance 40 in a viewing angle when observed from view-point 101.
Leakage of light 60 of illuminating light from clearance 41 between gate bus line 12 and pixel electrode 14 not shown exists.
As shown in FIG. 9, in LCD of this type of active matrix system, a liquid crystal layer 30 is interposed through alignment layers 16 and 24 between an array substrate 70 and a color filter substrate 80 positioned on the side of view-point 101 of an observer.
A plurality of parallely spaced source bus lines 11 and a plurality of parallely spaced gate bus lines (not shown) extending in the direction perpendicular to the direction of gate bus lines are provided in lattice shape on the array substrate 70. Each area partitioned by adjacent two source bus lines and adjacent two gate bus lines is pixel portion. One pixel electrode 14 and one switching element such as TFT (thin film transistor) are provided to each pixel portion. Colored layer 23 is provided on color filter substrate 80.
Colored layer 23 is partitioned by approximately the equal distance by black matrix (BM) 21 serving as light shield portion arranged in lattice shape, to become a pixel portion.
The illuminating light exists as a back light under the array substrate 70. BM 21 serving as a light shield portion of color filter substrate 80, when pixel of LCD 100 is seen from view-point 101 of observer, is required to prevent leakage of light 60 from the clearances 40 or 41 between source bus line 11 of array substrate 70 or gate bus line 12 and pixel electrode 14 in all of the pixels.
FIG. 10 is an observation view of LCD of active matrix system when a size of display area has become larger than that shown in FIG. 8 and FIG. 9. FIG. 11 is a sectional view of its pixel portion.
A viewing angle .theta. becomes larger, in the case of the observing the right and left ends or the upper and lower ends of the display area of LCD 100, as size of the display area is enlarged (see FIG. 10 and FIG. 11).
As shown in the sectional view of a pixel portion of FIG. 11, in each area of the right and left end portions (or upper and lower end portions) of the display area where viewing angle .theta. becomes larger, the width of BM 21 serving as a light shield portion of color filter substrate 80 has to be increased to shield leakage of light 60, thereby lowering aperture ratio of the pixel portion due to increase in width of BM 21.
In addition, leakage of light is caused by positional displacement in the formation of the pixel pattern of array substrate 70. The displacement is caused by mismatching the position of source bus lines and gate bus lines with respect to the standard position determined by a mark for aligning (alignment mark) which is used at the time when array substrate is exposed.
The pixel pattern (first pixel pattern) formed on array substrate 70 is conducted in a patterning step by a lens projection system. This is a system in which a light exposure is conducted by using lens, exchanging a plurality of reticles (small-sized masks).
The light exposure is conducted for each step by moving a stage for fixing the substrate.
The method is effective for use where a light exposing operation is repeated with the use of reticle of the same design for one array substrate 70, because step by step light exposure can be performed.
However, when substrates are exposed by such stepper system in order to produce LCD having a large-sized display area, it is necessary to form a pixel pattern on array substrate 70 by dividing the large-sized display area into a plurality of exposure area and by exposing the plurality of areas. At this time, if the mask pattern is displaced from a prescribed position, the pixel pattern on the array substrate 70 is also displaced from a prescribed designed position. The displacement of the mask pattern is caused by warp of a large-sized glass substrate for array substrate or color filter substrate, or by misalignment of a mask with respect of the standard position, the mask being used when exposing.
On color filter substrate 80, BM 21 is formed by light exposure of one time for entire surface, namely, the pixel patterns (second pixel pattern) are formed at the same time, thereby making the interval of pixel portions constant in the entire area of the display area.
When array substrate 70 and color filter substrate 80 are superposed oppositely, the positional relation between the pixel of array substrate 70 and the pixel of color filter substrate 80 become different for each exposure area, thereby causing luminance difference for each exposure area.
The luminance difference is caused by displacement of pixel pattern when disposing. The displacement causes variations of aperture ratio in each exposure area. Accordingly, the variations of aperture ratio result in the luminance difference.
When LCD is observed obliquely for luminance difference, the difference is considerably observed.
FIG. 12 shows a sectional view of a pixel portion of exposure area formed as designed in the pixel pattern on the side of array substrate 70.
In this case, in observation from the oblique direction as shown, leakage of light 60 from clearance 40 between source bus line 11 on the side of array substrate 70, and pixel electrode 14 is shielded by BM 21 formed on the side of color filter substrate 80.
FIG. 13 is a sectional view of a pixel portion in exposure area formed due to displacement in the left direction of the pixel pattern on the side of array substrate 70.
In this case, in observation from the left oblique direction as shown, leakage of light 60 from clearance 40 between the source bus line 11 on the side of array substrate 70, and pixel electrode 14 can not be shielded by BM 21 on the side of color filter substrate 80. Thus, luminance difference is caused with respect to exposure area formed without displacement to the pixel pattern of color filter substrate 80.
Although adverse effects caused due to leakage of light 60 from clearance 40 between source bus line 11 and pixel electrode 14 are described, the similar problems are caused even from leakage of light 60 from clearance 41 (not shown) between gate bus line 12 (not shown) on the side of array substrate 70, and pixel electrode 14.
To prevent difference in the luminance caused due to displacement in positional relation between the pixel pattern of color filter substrate 80 for each exposure area caused in this manner and the pixel pattern of array substrate 70, the width of BM 21 to be formed on color filter substrate 80 is required to be sufficiently enlarged.
That is, in the left, right and upper, lower ends of the display area, viewing angle becomes largest.
Therefore, the width of BM 21 in the left, right and upper, lower ends of the display area is required to be largest. Conventionally, the required largest width of BM 21 is applied to all BMs in the display area.
As described above, in the conventional LCD, leakage of light from clearance between the source bus line of the array substrate or the gate bus line, and the pixel electrode, even when the central portion of the display area smaller in the viewing angle from the given view-point position (normally the central portion front face of the display area) and even when the right, left or the upper, lower end portions of the display area when the viewing angle becomes larger, is seen. To prevent the leakage of light and difference in the luminance for each exposure area from being caused, it is required to cover and hide with BM having width sufficiently large on the color filter substrate side.
Therefore, when the pixel of the color filter substrate in the entire area of the display area is formed repeatedly in the same pixel construction, the width of the BM serving as the light shield portion is necessary to be larger than the width necessary in the pixel of the exposure area of the display area central portion, thereby causing such a defect as to lower aperture ratio of the pixel.
Accordingly, an object of this invention is to solve the problems of the conventional LCD and to provide LCD which can obtain higher aperture ratio even in a large-sized LCD where the viewing angle becomes larger, and can obtain higher quality of displaying quality constrained in adverse effects caused through the leakage of light.